1. Field of the Invention
The present invention relates to a microcrystalline series photovoltaic element having a high photoelectric conversion efficiency and which enables one to produce a high performance semiconductor device, representatively such as a high performance solar cells a high performance photosensor, or the like, where particularly said solar cell stably exhibits solar cell characteristics without being deteriorated even when continuously used outdoors over a long period of time. The present invention also relates a process for producing said photovoltaic element. The present invention further relates to a building material in which said photovoltaic element is used and a sunlight power generation apparatus in which said photovoltaic element is used.
2. Related Background Art
Various photovoltaic elements have been using not only as independent power sources of electrical equipments but also as alternate energy sources of daily power supply systems. However, for the photovoltaic elements used as alternate energy sources of daily power supply systems, they are still unsatisfactory particularly in terms of their cost per a unit quantity of generated power. In this connection, various studies have been conducting in order to develop improved photovoltaic elements. For instance, particularly with respect to the materials which play the most important role in photoelectric conversion, technical research and development of crystalline type photovoltaic elements, thin film type photovoltaic elements, and the like have been carrying out. The crystalline type photovoltaic element is meant a photovoltaic element having a photoelectric conversion member comprising a single crystalline silicon semiconductor material or a polycrystalline silicon semiconductor material. The thin film type photovoltaic element is meant a photovoltaic element having a photoelectric conversion member comprising an amorphous silicon-containing semiconductor material such as an amorphous silicon semiconductor material and an amorphous silicon-germanium semiconductor material; a microcrystalline silicon-containing semiconductor material such as a microcrystalline silicon semiconductor material and a microcrystalline silicon-germanium semiconductor material; an amorphous or microcrystalline silicon carbide semiconductor material; or a compound semiconductor material. For such microcrystalline silicon-containing semiconductor material, although various studies have been made, its reduction to practical use has not progressed as in the case of the crystalline or amorphous semiconductor materials.
Now, attention has been focused on a report by J. Meier et als., stating that a photovoltaic element (a solar cell) in which a microcrystalline silicon (xcexcc-Si) semiconductor material is used exhibits a good photoelectric conversion efficiency and is free of light-induced degradation [see, J. Meier et als., Mat. Res. Soc. Symp. Proc., vol 420. pp. 3-14, 1996 (hereinafter referred to as xe2x80x9cdocument 1xe2x80x9d)]. In document 1, there are described that said photovoltaic element (solar cell) was prepared by a high frequency plasma CVD process wherein glow discharge is caused in an atmosphere composed of silane gas diluted with a large amount of hydrogen gas (H2) by supplying a VHF (very high frequency) power with a frequency of 70 MHz therein and that the photovoltaic element is structured to have a p-i-n junction where the i-type semiconductor layer comprises a xcexcc-Si semiconductor material. Document 1 describes that the photovoltaic element afforded a photoelectric conversion efficiency of 7.7%, and no light-induced degradation was observed for the photovoltaic element Document 1 also describes that a stacked type photovoltaic element (solar cell) prepared by stacking a xcexcc-Si semiconductor material and another xcexcc-Si semiconductor material was found to have an initial photoelectric conversion efficiency of 13.1% and a relative light-induced degradation of 12.4%.
Besides, in K. Yamamoto et als., Jpn. J. Appl. Phys. vol. 33 (1994), pp. L1751-L1754, Part 2, No. 12B, Dec. 15, 1994 (hereinafter referred to as xe2x80x9cdocument 2xe2x80x9d), there is described a photovoltaic element (a solar cell) having a polycrystalline layer formed by subjecting a heavily boron-doped a-Si (amorphous silicon) p-type layer to excimer laser annealing and a pillar-like xcexcc-Si structure formed by way of plasma CVD on said polycrystalline layer.
However, the photovoltaic elements disclosed in documents 1 and 2 have such disadvantages as will be mentioned below.
Particularly, with reference to the description of document 1, it is understood that no light-induced degradation is observed for the microcrystalline photovoltaic elements disclosed therein. However, for the photovoltaic element for which no light-induced degradation was observed, it is understood that the xcexcc-Si active layer is of a thickness of 3.6 xcexcm which is relatively thick and that the short-circuit current of the photovoltaic element is 25.4 mA/cm2 and the photoelectric conversion efficiency thereof is 7.7% which is undesirably small. And it is also understood that in the formation of the xcexcc-Si active layer with such large thickness of 3.6 xcexcm, since the deposition rate is 1.2 xc3x85/sec which is slow, it takes about 8 hours in order to complete the formation thereof. In addition, for the stacked type microcrystalline photovoltaic element disclosed in document 1, although the initial photoelectric conversion efficiency thereof is 13.1% which is satisfactory, the photovoltaic element unavoidably suffers light-induced degradation upon repeated use where the initial photoelectric conversion efficiency is eventually deteriorated. And it obviously takes a long time for the preparation of the stacked type microcrystalline photovoltaic element.
With reference to the description of document 2, it is understood that the xcexcc-Si active layer of the photovoltaic element is of a thickness of 2 xcexcm, the short-circuit current of the photovoltaic element is 14.3 mA/cm2, and the photoelectric conversion efficiency thereof is 2.5% which is extremely small.
Separately, four persons of the group who reported document 2 jointly have developed the technique disclosed in document 2 and reported a thin film polycrystalline photovoltaic element (solar cell) formed by way of plasma CVD in which the active layer has a thickness of 3.5 xcexcm and which has a short-circuit current of 26.12 mA/cm2 and a photoelectric conversion efficiency of 9.8% (see, Kenji Yamamoto et als., 14th European Photovoltaic Solar Energy Conference, Barcelona. Spain, Jun. 30-Jul. 4, 1997, pp. 1018-1021). However, the photovoltaic element reported is still insufficient particularly in terms of the photoelectric conversion efficiency and the productivity.
Independently, it is known that a silicon thin film exhibiting crystalline properties may be grown from liquid phase by way of a casting method or the like. However, this method is disadvantageous in that high temperature treatment is required and that the method is not satisfactory particularly in terms of the productivity and production cost.
Besides, Japanese Unexamined Patent publication No. 109638/1993 discloses a method of forming a polycrystalline silicon film by subjecting an amorphous silicon film to a heat treatment so as to cause solid phase epitxaxy. Particularly, this publication describes a method in that a doped amorphous silicon film doped with P and a non-doped amorphous silicon film are sequentially formed on a substrate by means of a plasma CVD method, followed by subjecting to a heat treatment at about 600xc2x0 C. for several tens hours, where the doped amorphous silicon film is polycrystallized to have a grain size of more than several microns (xcexcm) and along with this, the non-doped amorphous silicon film is also polycrystallized to have a grain size of more than several microns (xcexcm), whereby a polycrystalline film is obtained. Japanese Unexamined Patent publication No. 136062/1993 discloses a method of forming a polycrystalline silicon film by repeating a step of forming a silicon film and exposing said amorphous silicon film to hydrogen plasma.
However, any of these methods is disadvantageous. That is, in the method described in the former publication, when a polycrystalline silicon semiconductor film having a thickness of more than several microns (xcexcm) is intended to form, it is necessary to conduct heat treatment for a long period of time because crystal growth takes place by way of solid phase reaction. Similarly, when a polycrystalline silicon semiconductor film having a thickness of more than several microns (xcexcm) is intended to form by the method described in the latter publication, the processing time for the formation of said semiconductor film is unavoidably prolonged because the step of forming a silicon film and exposing said amorphous silicon film to hydrogen plasma is repeated many times.
An principal object of the present invention is to eliminate the foregoing disadvantages in the prior art and to provide an improved microcrystalline series photovoltaic element which generates a large quantity of electric current and exhibits a high photoelectric conversion efficiency and a process for effectively producing said photovoltaic element.
Another object of the present invention is to provide an improved microcrystalline series photovoltaic element which exhibits a satisfactory photoelectric conversion efficiency even when the semiconductor layer is relatively thin and where the semiconductor layer can be formed at a high deposition rate, and which can be efficiently produced at low temperature and at a reasonable production cost, and a process for producing said photovoltaic element.
A further object of the present invention is to provide an improved microcrystalline series photovoltaic element which is stably exhibits excellent photovoltaic characteristics even when continuously used under sever environmental conditions over a long period of time, a process for producing said photovoltaic element, a building material in which said photovoltaic element is used, and a sunlight power generation apparatus in which said photovoltaic element is used.
A further object of the present invention is to provide, as aforesaid photovoltaic element, a microcrystalline series photovoltaic element having a stacked structure comprising a first semiconductor layer containing no crystalline phase therein, a second semiconductor layer containing approximately spherical microcrystalline phases, and a third semiconductor layer containing pillar microcrystalline phases which are stacked in this order, wherein the spherical microcrystalline phases of the second semiconductor layer on the side of the third semiconductor layer have an average size which is greater than that of those on the side of the first semiconductor layer and if necessary, the third semiconductor layer may have a layer region containing approximately spherical microcrystalline phases and pillar microcrystalline phases in a mixed state in the vicinity of the second semiconductor layer.
In the present invention, the term xe2x80x9cmicrocrystalline phasexe2x80x9d is meant a microcrystalline particle. The term xe2x80x9capproximately spherical microcrystalline phasexe2x80x9d is meant a microcrystalline particle shaped in an approximately spherical form including a polyhedral spherical form or the like. The xe2x80x9csize of the approximately spherical microcrystalline phasexe2x80x9d is meant a diameter of said approximately spherical microcrystalline particle. The term xe2x80x9cpillar microcrystalline phasexe2x80x9d is meant a microcrystalline particle shaped in a pillar form.
The present invention still makes it an object to provide a photovoltaic element having excellent photo-electric characteristics, which can be formed at a film-forming speed with a processing time of industrially practical level and at a reasonable cost.
A further object of the present invention is to provide a photovoltaic element comprising at least a first transparent electrically conductive layer formed on a substrate, a silicon series semiconductor layer having at least one p-i-n junction stacked on said first transparent electrically conductive layer, and a second transparent electrically conductive layer stacked on said silicon series semiconductor layer, characterized in that said silicon series semiconductor layer has a p-i-n junction structure which comprises a primary layer comprising an amorphous semiconductor layer having a first conduction type and a semiconductor layer containing crystalline phases (hereinafter referred to as xe2x80x9ccrystalline phase-containing semiconductor layer) and having a first conduction type sequentially stacked, a crystalline phase-containing i-type semiconductor layer, and a non-single crystal semiconductor layer having a second conduction type which are sequentially stacked, wherein the crystalline phase-containing semiconductor layer having the first conduction type contains crystalline phases (crystalline particles) such that their size magnitude is increased toward the crystalline phase-containing i-type semiconductor layer.
A further object of the present invention is to provide a process for producing the above-described photovoltaic element.
The primary layer is preferred to be formed such that a doped amorphous layer (that is, an amorphous layer doped with a prescribed dopant) and a non-doped amorphous layer are sequentially stacked to for a two-layered structure, and the two-layered structure is subjected to crystallization treatment, where part of the amorphous layer of the two-layered structure is crystallized. In this case, it is possible that part of the doped amorphous layer of the two-layered structure is crystallized. The crystallization treatment may be a laser radiation treatment or heat treatment.
In any case, the primary layer is preferred to have a Raman scattering intensity owing to the amorphous component thereof which is smaller than that owing to the crystalline component thereof.
It is preferred that the dopant concentration of the primary layer is decreased toward the crystalline phase-containing i-type semiconductor layer.
It is preferred to make the primary layer such that when the thickness of the crystalline phase-containing semiconductor layer having the first conduction type is made to be xe2x80x9cdxe2x80x9d and of the crystalline phases (crystalline particles) contained in said semiconductor layer, the length of a crystalline phase portion whose length being the longest is made to be xe2x80x9crxe2x80x9d, the value of r/d is less than 100.
It is preferred that the silicon series semiconductor layer is formed by means of a plasma CVD method using a high frequency power with a frequency preferably in a range of 10 MHz to 10 GHz.
It is preferred that the substrate comprises an electrically conductive substrate.